This invention relates to AC etching of aluminum foil for electrolytic capacitors, and in particular to a process utilizing alternating current and a chloride electrolyte bath containing tartaric and nitric acids.
AC etching has been used to produce aluminum articles such as lithographic plates and capacitor foil. It also has been used to electropolish aluminum articles prior to metal plating.
The prior art has discussed the difficulties in obtaining an even or uniform etch structure and has overcome these difficulties in a variety of ways, e.g., interrupting the etch process to apply protective coatings, carrying out the etch process in stages of differing degrees of aggressiveness, and using additives in the electrolyte bath to control pit size or to increase mechanical strength of the foil.
Another problem associated with prior art etching of aluminum foil has been to prevent aluminum hydroxide, formed during etching, from precipitating on or into the etched surfaces.
The resolution of these problems had led to prior art processes in which the etch conditions are carefully controlled to provide the desired increase in surface area and, particularly for capacitor foil, with little change in mechanical strength. Such a process is described by Arora and Randall in U.S. Pat. No. 4,279,714 issued July 21, 1981 that utilizes a phosphate in the hydrochloric etch bath. Another such process is described by Arora, Paquette, and McPherson in compending U.S. patent application Ser. No. 280,659 filed July 6, 1981 that utilizes phosphoric and nitric acids in the hydrochloric acid etch bath. These materials however must be removed from the etch structure before the foil is processed to make electrolytic capacitors.